The present invention generally relates to radiotelephone communication systems and more specifically to a method and apparatus for registration of radiotelephones in networked cellular radiotelephone systems each covering adjacent geographical areas.
Radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting by way of high-power transmitters to a limited number of mobile or portable radiotelephones in a large geographic area. To avoid repetition, the word "mobile" and "mobiles" will be used hereinafter to mean mobile or portable radiotelephones. Mobile transmissions, due to their lower power transmitters, were generally received in previous systems by a network of satellite receivers remotely located from the central site for receiving and relaying mobile transmissions to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the specific number of available channels.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the radio channels in a metropolitan area, such as the metropolitan area of a large city like Chicago or New York, by dividing the radio coverage area into smaller coverage areas called cells using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area enables the radio channels used in one cell to be reused in another cell geographically separated according to a predetermined plan, such as a four cell pattern shown and described in U.S. Pat. No. 4,128,740. In this four cell pattern, each cell is assigned a subset of the available radio channels and reuse of the radio channels is accomplished by repeating the pattern throughout a metropolitan area.
A cell system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one paging/access channel and several voice channels. The paging/access channel is dedicated to controlling the operation of the mobiles by means of data messages transmitted to and received from the mobiles. Control functions performed include paging selected mobiles, receiving requests for service from mobiles, instructing mobiles to tune to a voice channel where a conversation may take place, identifying the particular cellular system to the mobiles and enabling mobile registration by which process the mobiles identify themselves to the system. The data message and radio channel specifications for U.S. cellular radiotelephone systems are set forth in Electronic Industries Association (EIA) Interim Standard IS-3-C implemented in accordance with 47 CFR 22 and the Report and Orders pertaining to Federal Communications Commission Docket 79-318. Copies of EIA Interim Standard IS-3-C may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Eye Street, N.W., Washington, D.C. U.S.A. 20006.
Since the cells may be of relatively small size, the likelihood of a mobile travelling out of one cell and into another cell is high. The process of switching the established call from one cell, the source cell, to another, the target cell, is known as handoff. The cellular system determines the need for a handoff by periodically measuring the signal strength of each active mobile. If the measured signal strength is below a predetermined level, the cellular system determines the availability of other channels in neighboring target cells and composes an instruction to the mobile commanding it to tune to an available channel in an acceptable target cell. As more cellular systems are installed, handoffs between different cellular systems may take place in order to maintain a call as the mobile passes from the coverage area of one cellular system to another.
Since mobiles may quickly move between cells and adjacent cellular systems, terminating a call to a particular mobile may necessitate paging that mobile in all cells of the particular cellular system where the mobile is currently located. Mobile location information may be obtained by requiring that each mobile register frequently with its cellular system. However, frequent updating of each mobile's location will divert processing capacity of its cellular system from call processing to registration processing, thereby reducing the number of calls that may be processed and adversely impacting revenues. Accordingly, there is a need for an improved method and apparatus for registration of radiotelephones in networked cellular systems in order to optimize call processng capacity and revenues of the networked cellular systems.